1. Field of the Invention
The present invention relates to an optical pickup device for emitting a beam of light to read information from a storage medium.
2. Description of the Related Art
An optical pickup device is adapted to operate under the focusing servo control and the tracking servo control to accurately read information stored on the tracks of an optical storage medium such as an optical disk. The focusing servo control is adapted to move an objective lens back and forth relative to the storage surface of the storage medium such that a beam of light for illuminating the storage medium is focused on the storage surface. On the other hand, the tracking servo control is adapted to translate the objective lens in a direction perpendicular to the tangential direction of the track such that the beam of light is positioned on the track of the storage surface.
The focusing servo control generates a focus error signal indicative of the degree of focus error in accordance with the level of received light reflected from the storage medium and allows a focusing actuator to drive the objective lens back and forth in order to reduce the focus error signal.
The tracking servo control generates a tracking error signal indicative of the degree of tracking error in accordance with the level of received light reflected from the storage medium and allows a tracking actuator to drive the objective lens in the radial direction of the optical disk in order to reduce the focus error signal.
For example, the methods for generating the focus error signal in the focusing servo control include the so-called spot size method or astigmatism method, while the methods for generating the tracking error signal in the tracking servo control include a so-called three-beam method.
In the spot size method and the three-beam method, a diffraction grating, a hologram or the like is employed to split a single beam of light into one diffracted beam of 0th order and two diffracted beams of +/xe2x88x92 first order. Of these beams of light, the beam of 0th order is called the main beam, while the beams of +1st and xe2x88x921st order are called the side beams.
It has been generally practiced to obtain the focus error signal by the main beam in an optical pickup device that employs a plurality of beams of light as well as in an optical pickup device employing the three-beam method. That is, the side beams other than the main beam are subjected substantially in vain to optical action or would rather have a harmful effect such as crosstalk between optical beam signals on a light-receiving element of an optical detector (hereinafter referred to as the detector). For example, in a generally organized differential size spot method, a hologram element is placed at a portion where the three beams are not separately available, thereby making two light beams having different focal lengths available. The three-beam method provides for a total of 6 light beams (=3xc3x972). This causes the number of optical beam spots to increase on the detector, whereby an additional number of subdivisions (light-receiving elements) are required of the detector corresponding to the increase.
For example, such an optical pickup device has been suggested which employs the three-beam method as disclosed in Japanese Patent Laid-Open Publication No. Hei 8-55363.
As shown in FIG. 1, the optical pickup device disclosed therein includes a module which is integrated with a laser detector and has a hologram element formed of parallel flat plates. A divergent beam 63b emitted from a semiconductor laser 1b is reflected upon a mirror 82 and incident on the hologram element. The hologram element is provided with a hologram 17 and an annular grating 83. The light beam incident on the hologram element passes through the annular grating 83 and is then split into three beams of 0th and +/xe2x88x92 1st orders, whereby the main and side beams for tracking are generated.
The three beams pass through the hologram 17 and then an objective lens (not shown) to converge each as an optical spot on an optical disk (not shown).
Each optical spot reflected and diffracted on the optical disk passes again through the objective lens to be then incident on the hologram element as a return light beam. Each of the return light beams incident on the hologram element passes through the hologram 17 to be then split into three beams. This yields six beams in total, or diffracted beams of +/xe2x88x92 1st orders are received by an optical detector 13b and converted into electrical signals. Then, the electrical signals delivered by the optical detector 13b is operated as desired, thereby making it possible to provide a focus error signal, a tracking error signal, and a written information signal.
In the prior art disclosed in Japanese Patent Laid-Open Publication No. Hei 8-55363, the hologram 17 is placed before where the main and side beams of the return light are split physically and spatially. Accordingly, the optical pickup device is optically designed such that, of the nine beams obtained by splitting the main and side beams of the return light, specific diffracted beams are made available. In this prior art, the three-beam method also provides six optical beam spots in total, resulting in an increase in number of the subdivisions of the detector. The optical design of the hologram 17 is also made complicated.
The optical pickup device cannot be made large for use with a storage medium read device such as an optical disk player. It is therefore desired to make compact the portions related to the focusing servo control and the tracking servo control.
In view of the aforementioned problems, an object of the present invention is to provide a compact optical pickup device which can accomplish the focusing servo control and the tracking servo control with stability.
The optical pickup device according to the present invention includes an illuminating optical system for focusing a light beam, split in a main beam and at least one side beam, onto a track on an information storage surface of an optical storage medium to form optical spots thereon. The device also includes a light detecting optical system for introducing return light reflected back from the information storage surface, and a polarizing optical element. The polarizing optical element has regions split at the center of an optical path by a parting line extending at least either in a direction of extension of the track or in a direction perpendicular to the direction of extension. The polarizing optical element also splits the main beam return light at least in two for each of the regions on a plane perpendicular to the optical path of the return light of the reflected main beam in the light detecting optical system. The polarizing optical element is disposed where the return light of the main and side beams is spatially separated. The optical pickup device further includes an optical detector disposed in contact with the polarizing optical element. The optical detector has a plurality of main-beam light-receiving elements for receiving the separated main beam return light and a plurality of side-beam light-receiving elements for receiving the separated side beam return light.
In one aspect of the optical pickup device according to the present invention, said polarizing optical element comprises a parallel plate portion formed of an optically transparent material, and a split reflecting surface, formed on a side of light emission of said parallel plate portion, for reflecting return light in said split regions, and
said main and side beam light-receiving elements are formed on the side of light emission of said parallel plate portion, and two or more main-beam light-receiving elements are disposed to allow said separated main beam return light to be reflected at a boundary surface on a side of light incidence of said parallel plate portion to reach said main-beam light-receiving element.
In another aspect of the optical pickup device according to the present invention, said split reflecting surface is a pyramidal prism recessed surface having a ridge line corresponding to said parting line.
In a further aspect of the optical pickup device according to the present invention, said split reflecting surface is a roof type prism recessed surface having a ridge beam line corresponding to said one parting line and a step portion along said other parting line.
In a still further aspect of the optical pickup device according to the present invention, said split reflecting surface is a hologram for reflecting light in different directions in said split regions.
In another aspect of the optical pickup device according to the present invention, said split reflecting surface is provided with a semitransparent film.
In a further aspect of the present invention, the optical pickup device further comprises:
a three-way-split detector, disposed opposite to said split reflecting surface, having three light-receiving regions split by two parallel parting lines extending in a direction perpendicular to the direction of extension of said track,
wherein each of said main-beam light-receiving elements comprises two light-receiving regions split by two-way-splitting parting lines extending generally in parallel to a direction perpendicular to the direction of extension of said track,
said two-way-splitting parting line extends to where signals delivered by said two light-receiving regions derived from said return light spot when a light beam is focused on an optical storage medium are generally equal to each other, and
a differential spot size method is employed to generate a focus error signal in accordance with an output of said three-way-split detector and said main-beam light-receiving elements, said focus error signal consists of the sum of differences between signals delivered from the two light-receiving regions of said main-beam light-receiving elements and the sum of differences between outputs from a light-receiving region sandwiched by two light-receiving regions of said three-way-split detector and from the sandwiching two light-receiving regions.
In a still further aspect of the optical pickup device according to the present invention, tracking servo control is provided by a phase difference method for detecting a phase difference in each sum signal delivered from said main-beam light-receiving elements for receiving independently said return light having passed through said split regions.
In another aspect of the optical pickup device according to the present invention, said hologram splits said return light in two or more for each of said regions and has an optical function for providing said return light passing through said regions adjacent to each other on the same side split by said parting line with astigmatisms rotated by 90 degrees relative to each other around the optical path,
each of said main-beam light-receiving elements comprises two light-receiving regions split by two-way-splitting parting lines extending generally in parallel to said parting line disposed in a direction perpendicular to the direction of extension of said track,
said two-way-splitting parting line extends to where signals delivered by said two light-receiving regions of said light-receiving elements derived from said return light spot received on said light-receiving elements on an image plane where a light beam becomes circular in an optical system provided with astigmatism are generally equal to each other, and
a focus error signal is generated from the sum of differences between signals delivered from the two light-receiving regions of said light-receiving elements.
In a further aspect of the optical pickup device according to the present invention, said polarizing optical element comprises a parallel plate portion formed of an optically transparent material, and a four-way-splitting hologram, formed on a side of light incidence of said parallel plate portion, for splitting in four and transmitting return light in said split regions,
said main and side beam light-receiving elements are formed on the side of light emission of said parallel plate portion, and four of said main-beam light-receiving elements for receiving the return light of said split main beam are disposed apart from each other in said split regions,
said hologram splits said return light in four for each of said regions and has an optical function for providing said return light passing through said regions adjacent to each other on the same side split by said parting line with astigmatisms rotated by 90 degrees relative to each other around the optical path,
each of said main-beam light-receiving elements comprises two light-receiving regions split by two-way-splitting parting lines extending generally in parallel to said parting line disposed in a direction perpendicular to the direction of extension of said track,
when a light beam becomes circular in an optical system provided with astigmatism, said two-way-splitting parting line extends to where signals delivered by said two light-receiving regions of each of said main beam light-receiving elements derived from said return light spot received on said main beam light-receiving elements are generally equal to each other, and
a focus error signal is generated from the sum of differences between signals delivered from the two light-receiving regions of each of said main beam light-receiving elements.
In a still further aspect of the optical pickup device according to the present invention, said polarizing optical element comprises a two-way-splitting hologram, formed on the side of light incidence of said parallel plate portion, for splitting in two and transmitting return light of said side beam for each of said split regions split in two along a parting line extending in the direction of extension of said track.
In another aspect of the optical pickup device according to the present invention, each of said side-beam light-receiving elements comprises two light-receiving regions spaced apart from each other and split in two along a parting line extending in the direction of extension of said track, and tracking servo control is provided by a differential push-pull method in accordance with a sum signal of differences between signals delivered from the two light-receiving regions of each of said side-beam light-receiving elements.